1. Field of Invention
This invention relates to golf balls and, more particularly, to golf balls having covers made of polyurethane.
2. Discussion of Prior Art
Conventionally, golf ball covers are formed about cores in a one-step molding process by either compression molding or injection molding in which cover stock is molded about a core. The core is either wound or solid. Solid cores are generally made from a homogeneous mass of material, while wound cores are made by winding elastic thread about a center. The center is typically either a small balloon filled with liquid or a solid mass of material.
In compression molding, a core is placed between two relatively smooth, preformed half-shells made from a cover stock and then placed into a compression mold. A compression mold generally comprises two mold plates, each of which houses a plurality of mold cups. Each mold cup has a negative dimple pattern which is embossed into the cover half-shells by closing the mold and subjecting the core and cover stock to heat and pressure. The heat and pressure cause the preformed cover half-shells to soften and flow about the core to pick up the dimple pattern of the mold cups.
With injection molding, cores are placed in a mold in which the mold cups are typically equipped with retractable pins. The pins ensure correct positioning of the core in the cup. Once the core is correctly positioned in the closed mold, cover stock is injected into the mold cups and flows about the core. The cover stock picks up the dimple pattern of the mold cups. Once the mold cups are filled with cover stock, the pins are retracted while the cover stock is still at least semi-flowable, allowing the cover stock to close the pinholes. Retractable-pin molds are expensive, difficult to operate, often result in surface defects, and limit the number of compositions which can be used as cover stock.
There are only a few materials which are considered suitable for golf ball covers. Balata, a naturally occurring material which can also be made synthetically, was the material of choice until the mid-1960's. Balata was known for its great "click" and "feel" but was also known to "cut like butter." In the 1960's, E. I. du Pont de Nemours & Company came out with a new plastic product, an ionomer which was sold under the name SURLYN. SURLYN resin was soon tried on golf balls and, because of its great toughness, won an increasing share of the market to the point that, as of 1988, it has been used in more than 90% of all golf balls sold. While SURLYN has been a remarkable success as a golf ball cover material, it is hard as compared to balata and many golfers continue to prefer balata because of its "click" and "feel." SURLYN.RTM. is a trademark for a group of ionomer resins which are generally copolymers of an olefin and an alpha, beta ethylenically unsaturated carboxylic acid with 10-90% of the carboxylic acid groups neutralized by metal ions.
The search has continued for a golf ball cover material with the durability of SURLYN and the "click" and "feel" of balata. Polyurethanes have been considered periodically for quite a few years because they meet the requirements of both durability and good "click" and "feel"; the big disadvantage to polyurethanes, however, has been processability. While golf ball covers can be formed from polyurethanes, and, indeed, there is considerable patent literature in this area (see, for example, U.S. Pat. No. 3,989,568), commercial processability has continued to be a substantial problem.
Polyurethanes are the result of a reaction between a prepolymer and a polyfunctional curing agent, notably a polyamine or a polyol. The prepolymer is made from a polyether or a polyester with a diisocyanate, polyethers being preferred because of their water resistance. Both thermoplastic and thermosetting polyurethanes are known; however, thermosets have been found to produce better cover stocks for golf balls. A thermosetting material is a polymer that "sets" irreversibly. The setting of a polyurethane is accomplished by cross-linking of the prepolymer with the polyfunctional curing agent.
U.S. Pat. No. 3,177,280, issued April 6, 1965, teaches using a two-step process for making a polyurethane covered golf ball by a first step of forming a smooth cover about a core and then a second step of overmolding a dimple pattern into the still soft, partially cured urethane cover of the smooth covered golf ball. The first step is accomplished by either transfer molding cover stock about a core in a closed mold or by using a doctor blade to coat the exterior of a cover with cover stock material. Transfer molding of cover stock into a closed mold has led to a number of surface defects which make the golf ball unacceptable. Using a doctor blade to form the cover requires equipment not readily available on the open market.